Limit for rescoring factor is 1.0, so we can't have less rescored docs than num_candidates

Author: carlosdelest

server/src/main/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapper.java

@@ -2113,11 +2113,8 @@ && isNotUnitVector(squaredMagnitude)) {
 
             int adjustedNumCands = numCands;
             if (needsRescore(numCandsFactor)) {
-                // We shouldn't have less than k candidates (or 1 in case k is not set) to rescore
-                int minCands = k == null ? 1 : k;
                 // k <= numCands * numCandsFactor <= NUM_CANDS_OVERSAMPLE_LIMIT. Adjust otherwise.
-                adjustedNumCands = Math.max(minCands, (int) Math.ceil(numCands * numCandsFactor));
-                adjustedNumCands = Math.min(adjustedNumCands, NUM_CANDS_OVERSAMPLE_LIMIT);
+                adjustedNumCands = Math.min((int) Math.ceil(numCands * numCandsFactor), NUM_CANDS_OVERSAMPLE_LIMIT);
             }
             Query knnQuery = parentFilter != null
                 ? new ESDiversifyingChildrenFloatKnnVectorQuery(name(), queryVector, filter, k, adjustedNumCands, parentFilter)

server/src/main/java/org/elasticsearch/search/vectors/RescoreVectorBuilder.java

@@ -24,7 +24,7 @@
 public class RescoreVectorBuilder implements Writeable, ToXContentObject {
 
     public static final ParseField NUM_CANDIDATES_FACTOR_FIELD = new ParseField("num_candidates_factor");
-    public static final float MIN_OVERSAMPLE = 0.0F;
+    public static final float MIN_OVERSAMPLE = 1.0F;
     private static final ConstructingObjectParser<RescoreVectorBuilder, Void> PARSER = new ConstructingObjectParser<>(
         "rescore_vector",
         args -> new RescoreVectorBuilder((Float) args[0])
@@ -39,8 +39,8 @@ public class RescoreVectorBuilder implements Writeable, ToXContentObject {
 
     public RescoreVectorBuilder(Float numCandidatesFactor) {
         Objects.requireNonNull(numCandidatesFactor, "[" + NUM_CANDIDATES_FACTOR_FIELD.getPreferredName() + "] must be set");
-        if (numCandidatesFactor <= MIN_OVERSAMPLE) {
-            throw new IllegalArgumentException("[" + NUM_CANDIDATES_FACTOR_FIELD.getPreferredName() + "] must be > " + MIN_OVERSAMPLE);
+        if (numCandidatesFactor < MIN_OVERSAMPLE) {
+            throw new IllegalArgumentException("[" + NUM_CANDIDATES_FACTOR_FIELD.getPreferredName() + "] must be >= " + MIN_OVERSAMPLE);
         }
         this.numCandidatesFactor = numCandidatesFactor;
     }

server/src/test/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldTypeTests.java

@@ -462,10 +462,6 @@ public void testRescoreOversampleModifiesNumCandidates() {
         // Oversampling limits for num candidates
         checkRescoreQueryParameters(fieldType, 1000, 1000, 11.0F, 1000, 10000);
         checkRescoreQueryParameters(fieldType, 5000, 7500, 2.5F, 5000, 10000);
-        // Oversampling is capped at k as a minimum
-        checkRescoreQueryParameters(fieldType, 10, 100, 0.01F, 10, 10);
-        // Oversampling is capped at 1 as a minimum if k is not specified
-        checkRescoreQueryParameters(fieldType, null, 100, 0.0001F, null, 1);
     }
 
     private static void checkRescoreQueryParameters(

server/src/test/java/org/elasticsearch/search/vectors/KnnSearchBuilderTests.java

@@ -257,9 +257,9 @@ public void testInvalidK() {
     public void testInvalidRescoreVectorBuilder() {
         IllegalArgumentException e = expectThrows(
             IllegalArgumentException.class,
-            () -> new KnnSearchBuilder("field", randomVector(3), 10, 100, new RescoreVectorBuilder(0.0F), null)
+            () -> new KnnSearchBuilder("field", randomVector(3), 10, 100, new RescoreVectorBuilder(0.99F), null)
         );
-        assertThat(e.getMessage(), containsString("[num_candidates_factor] must be > 0.0"));
+        assertThat(e.getMessage(), containsString("[num_candidates_factor] must be >= 1.0"));
     }
 
     public void testRewrite() throws Exception {